The present invention relates generally to functional neuromuscular stimulation (FNS) or functional electrical stimulation (FES) and, more particularly, to a closed-loop electrical control system for effecting and controlling such stimulation.
It is well known that spinal cord injuries can paralyze motor-function muscles by cutting off the transmission of neural information between the central nervous system and the peripheral nervous system in portions of the body below the level of the lesion in the spinal cord. Muscles used for motor functions can also be paralyzed by other types of events, such as cerebral vascular accidents.
It is also well known that partial restoration of the motor functions of such paralyzed persons can be achieved by functional electrical stimulation of the paralyzed muscles, in both upper and lower limbs. These FES systems use electricity to evoke a muscle response by the use of electrical stimulation as a substitute for the missing neural signals. The electrical stimulation can be effected by surface electrodes, percutaneous intramuscular electrodes, or implanted electrodes.
The electrical signals supplied to the stimulating electrodes in FES systems have been controlled by any of three different types of systems which are described in Cybulski, G. R., Penn, R. D. and Jaeger, R. J., "Lower Extremity Functional Neuromuscular Stimulation in Cases of Spinal Cord Injury", Neurosurgery 15:132-146, 1984. The first of these systems is an open-loop control system which permits the user to generate a command signal by means of a push button, rocker switch, hand-held manipulator or the like. The other two systems are both closed-loop control systems, one of which is referred to as "user-transparent", and the other as "supplemental sensory feedback".
In general, closed-loop control is preferred over open-loop control for both physiological and technological reasons. For example, there is a highly nonlinear relationship between the parameters which control the muscle stimulation and the resulting force and/or position output produced by the stimulated muscle. Moreover, the relationship between electrical stimulation of a muscle and the resulting output force is a time-dependent relationship. In open-loop systems the user must visually monitor the changes in force and/or position produced by the stimulation and adjust the command signal accordingly, and making these adjustments is both difficult and tedious. With closed-loop feedback control, however, the relationship between the input and output parameters can be automatically regulated to provide a linear relationship and to compensate for those effects which vary with time.
Closed-loop control systems for FES have been previously used only in laboratory experiments. In these systems, a sensor is required to produce the desired feedback signals in response to some physical consequence of the stimulation, such as force, position, stiffness or slip. For example, such feedback signals have been produced by the use of external sensors, i.e., sensors which are not implanted. External sensors, however, are typically difficult to calibrate, require frequent recalibration, are sensitive to environmental factors such as moisture and temperature, lead to breakage problems, and are bulky and unsightly and thus cosmetically undesirable.